Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Sep 3, herbaceous, somewhat unequal, 2 sometimes connate at base; pet 3, the upper 2 usually blue, ovate to reniform, clawed at base, the lower one smaller, in most spp. white; fertile stamens 3, 2 with oblong anthers, one incurved with a longer anther; sterile stamens (2)3, smaller, with imperfect, cruciform anthers; filaments not hairy; ovary sessile, 3-locular, the lower 2 locules fertile, with 1 or 2 ovules, the upper median locule smaller, 1-ovuled or empty, or abortive; fr a loculicidal, 2–3-valved capsule; herbs with succulent stem and linear to lance-ovate lvs; infl of small cymes, each closely subtended by a folded cordate spathe from which the pedicels protrude (or sometimes 2 cymes to a spathe); usually self-pollinated. 200+, cosmop., mostly warm reg.